Electro-chemical passivation of tinplate



United States Patent 3,491,001 ELECTRO-CHEMICAL PASSIVATION OF TINPLATE Ralph Edward Shackleford, Ancaster, Ontario, and Karl Stanley Chalupka, Hamilton, Ontario, Canada, assignors to The Steel Company of Canada, Limited, Hamilton, Ontario, Canada, a company of Canada No Drawing. Filed Oct. 31, 1966, Ser. No. 590,500 Int. Cl. C23b /50, 11/00 US. Cl. 204-56 Claims ABSTRACT OF THE DISCLOSURE A method of passivating tin or tin-base alloy surfaces in which said surfaces are subjected to a cathodic treatment in an aqueous solution containing alkaline, dichromate and chromate ions in amount stoichiometrically equivalent to from about to about 30 g.p.l. of sodium dichromate and from about 3 to about 6 g.p.l. of chromium trioxide at a sodium dichromate to chromium trioxide ratio within the range of from about 4:1 to about 7:1' and a pH below 2, preferably about 1. An anodic pretreatment of said surfaces in an aqueous solution containing from about 0.5 to about g.p.l. of an alkaline carbonate extends the upper limit of chromium trioxide to about 11 g.p.l. and broadens the sodium dichromate to chromium trioxide ratio range to from about 1.5 :1 to about 7:1.

This invention relates to corrosion inhibition of tin and tin-base alloy surfaces. More particularly, it relates to the provision of a composition and method of rendering tin and tin-base alloy surfaces stain and corrosion resistant while providing an improved base for lacquer and other organic coatings.

Conventional protection against oxidation and staining or de-tinning attack of tinplate surfaces has heretofore been accomplished by treatment with solutions of chromic acid and/or its salts. For example, cathodic treatments of tinplate in solutions of sodium dichromate are known, such solutions normally having pH levels or 4 or higher. Such treatments have not been completely satisfactory in that tinplate so treated was still susceptible to staining by sulphur-bearing materials, especially food products such as beans or pork, or to de-tinning, whereby prolonged exposure to food products such as vegetable soup or vegetable juices resulted in dissolution of the tin with exposure of blackened areas of iron-tin alloy.

The principal objects of the present invention are to provide a method for treating and coating tin and tinbase alloy surfaces, and the coating composition produced thereby, to render said surfaces stain and corrosion resistant; provide an improved base for lacquer, enamel or other organic base coatings, or obviate the necessity for such ancillary protective coatings; permit reduction in tin coating thicknesses; and provide a facile method positive in operation which will quickly produce a coating under high-speed line conditions.

These and other objects and the manner in which they can be attained will become apparent from the following detailed description of the method and composition of the invention.

We have discovered that tin and tin-base alloy surfaces can be effectively provided with stain and corrosion resistant coatings superior to known coatings by making said surfaces cathodic in an aqueous solution containing alkaline ions, dichromate ions and chromate ions. The form in which the ions are introduced does not appear critical provided the ions remain in solution in the correct proportions and the solution has the proper pH. We have found that an alkaline dichromate such as sodium-, potassiumor ammonium-dichromate with chromium trioxide or an alkaline hydroxide such as sodium-, potassiumor ammonium-hydroxide with chromic acid, provides a satisfactory source of the alkaline, dichromate and chromate ions. In carrying out the process of the invention, said ions should be present in amounts stoichiometrically equivalent to from about 15 to about 30 grams per litre (g.p.l.) of sodium dichromate and from about 3 to about 6 g.p.l. of chromium trioxide at a solution pH of about 1; the ratio of sodium dichromate to chromium trioxide being within the range of from about 4:1 to about 7:1. The surfaces to be coated should be made cathodic for at least about 0.2 second at a current density of at least about 20 amperes per square foot at a solution temperature of from about 18 C. to about C.

We have also discovered that the stain and corrosion resistance properties of said coatings are further enhanced, and the composition parameters of the cathodic treatment broadened, by a preliminary step comprising making said surfaces anodic for at least 0.1 second in an aqueous solution containing from about 0.5 to about 20 g.p.l. of an alkaline carbonate such as sodium carbonate at a current density of from about 20 to about 40 amperes per square foot and at a solution temperature of from about ambient temperature up to about 100 C.

In the operation of the method of the invention, tinplate or plate having other tin-base alloy coatings thereon of from 0.125 to 1.0 lb./bb. weight, is subjected to an anodic treatment of the type described generally above, and which will be described in more detail hereinbelow, if a preliminary anodic treatment is desired. If no anodic pretreatment is employed, the surface to be coated is immersed in an aqueous solution containing alkaline, dichromate and chromate ions and amounts stoichiometrically equivalent to from about 15 to about 30 g.p.l. of sodium dichromate and from about 3 to about 6 g.p.l. of chromium trioxide; the ratio of sodium dichromate t0 chromium trioxide preferably being within the range of 4:1 to 7:1. We have found that this ratio provides sufficient chromic acid in the solution to maintain the solution pH below 2, preferably at about 1, it being desirable for optimum coating efiiciency of the method to maintain the pH below 2 by the addition of chromium trioxide to the solution. We have also found that optimum results are obtained with the presence of alkaline, dichromate and chromate ions in the aqueous solution equivalent to 20 g.p.l. of sodium dichromate and 5 g.p.l. of chromium trioxide at a sodium dichromate-chromium trioxide ratio of about 4:1, as will be evident from Examples 1 and 2 below.

The temperature of the aqueous solution is not critical and can be within the range of ambient room temperature of about 18 C. up to the boiling point of about 100 C. without appreciably affecting the operation of the method or the coating produced thereby.

The current density should be maintained within the range of from about 20 amperes per square foot to about 100 amperes per square foot, preferably not more than 50 amperes per square foot. A current density below about 20 amperes per square foot does not permit formation of a hard, abrasion-resistant coating and a current density above about 50 amperes per square foot does not improve the characteristics of the coating or the operation of the method. Current densities of about 100 amperes per square foot have been found to etch the strip being coated.

The following examples illustrate the preferred sodium dichromate and chromium trioxide contents of the treating solution, it being understood that the examples are illustrative only of the method of the invention and represent stoichometrically the amounts of alkaline, dichromate and chromate ions necessary in the solution in the coating of tinplate or other tin-base alloy surfaces not having received the anodic pretreatment. It will also be understood that the alkaline ions will be selected from the amperes per square foot. In general, not Only was resistance to de-tinning improved but also the tinplate had a uniform surface free of etching, film streaks or blooming at a sodium dichromate to chromium trioxide ratio within the range of from 4:1 to about 7:1 and curgroup consisting of alkali metal ions including sodium, 5 rent density above 20 amperes per square foot. and otassium; and ammonium.

p EXAMPLE 2 EXAMPLE 1 A number of tests were conducted to determine the A number of tests were conducted to determine the inpreferable sodium dichromate to chromium trioxide ratio fiuence of the dichromate to chromium trioxide ratio, the within the ratio limits of 4:1 to 7:1. The combined ininfluence of the anodic pretreatment step being obviated by fiuence of solution temperature and applied current dendeletion of this step. A wide range of sodium dichromate sity was determined for each solution ratio. All samples and chromium trioxide ratios, together with a wide range of treated tinplate were subjected to the standard de-tinof current densities, were covered; the maximum value ning test described in Example 1 above. of current density being dependent on the concentration It was found that a sodium dichromate to chromium ratio for a standard size sample. trioxide ratio of about 4:1 produced the optimum coating The tinplate treated was 1 to 0.25 lb./bb. reflowed and characteristics. Solution temperature Within the range of chemically untreated. The sodium dichromate and chrofrom about to about 100 C. proved to have no mium trioxide were of A.C.S. purity. The treatment 20 efiect on the process efiiciency. A minimum current denbasically consisted of acetone solvent cleaning followed sity of about 20 amperes per square foot was found by cathodic treatment for three seconds in the various necessary to yield a satisfactory product. Increases of solutions, after which excess solution was removed by current density above about 50 amperes per square foot rinsing in distilled water. The thus treated samples were appeared to offer no advantage in the operation of the exposed to a de-tinning medium consisting of unprocessed process. vegetable soup by immersing in said soup for 18 hours The foregoing examples describe tests in which the at a soup temperature of 115 C. and pressure of 15 surfaces coated were not subjected to apreliminary anodic p.s.i.g., which approximates commercial pack conditions, treatment. We have found that the anodic pretreatment Table I below shows the effect of variation of odi m described briefly above, in combination with the cathodic dichromate-chromium trioxide ratio and variation of curtreatment, results iI1 the Production of a further p rent density; the Series A tests indicating the effect of ingly improved Coating, as will be $6611 from Example sodium dichromate to chromium trioxide ratios of from 1:1 to 1:8, and the Series B tests indicating the eifect EXAMPLE 3 of sodium dichromate to chromium trioxide ratios of The effects of an anodic cleaning in an alkaline solufrom 1:1 to 8:1. tion such as sodium carbonate was studied in a number TABLE I Na2Cr2O Relative ClOs CIOa resistance to g.p.l. Ratio C.D. .A./ft. de-tinning Remarks 1 11 5 1:5 5 1:5 10 1:10 Surface etch. 10 1:10 Surface etch tendency less. 13 Surface uniformity apparent. 10 1:3: 3 Uniform surface. 10 1=a3 Do. 15 1:5 D0. 15 115 Do. 20 1:6.7 Do. 20 1:6. 7 D0. 22% 1 Surface film Streaky. 20 155 20 1:5 Blotgnirig tendency more pronounced 20 115 20 114 Do. 20 1:4 D0. 20 1:4 D0. 1=s D0. 40 1:8 D0. 40 1:8 Do. 40 1:8 Do.

1 1:1 6 d0 1 5=1 9 do. 1 10:1 12 do 2 511 18 .-do 3 3.311 24 .do 3 5:1 24 .....(1o. Uniform surface. 3 6.7:1 30 Good Do. 4 5:1 30 Do. 5 4:1 D0. 5 8:1 Do. 5 s=1 Do.

It will be noted that a definite increase in de-tinning resistance was observed at a sodium dichromate to chromium trioxide ratio within the range of from 4:1 to 6.7:1 at a current density of 30 amperes per square foot, the occurrence of a uniform surface coating being obof tests in which the pretreated tinplate surface was in each case subsequently cathodically treated in an aqueous solution of sodium dichromate and chromium trioxide in preferred amounts of 20 g.p.l. and 5 g.p.l. respectively at a served on specimens coa ed at a current density of 24 solution temperature of about 20 C, Table II below shows the relative effects of the pretreatment on the finished product.

CD. 40 A/ft. Cathodic 5 secs Do. CD. 40 A/ft. Anodie 1 see Do. C.D. 40 A/ft. Anodic 3 secs. Do. C.D. 40 Do.

A minimum of anodic treatment in a 20 g.p.l. solution of Na CO at current density of from 20 to 40 amperes per square foot greatly increases the efficiency of the subsequent chemical treatment stage. Cathodic treatment or simple immersion without applied polarity of the tinplate in the alkaline solution results in a relatively poor product.

It will be evident from Table II that, on a relative basis, anodic pretreatment of the tinplate subsequently coated results in the production of a consistently good coating on the tinplate; cathodic treatment without an anodic pretreatment results in a fair to good coating; and immersion in the pretreatment solution without applied current polarity, followed by cathodic treatment, results in a relatively poor product. Reduction of the sodium carbonate concentration to as low as 0.5 g.p.l. has permitted the production of a satisfactory coating while current densities in the 20 to 40 amperes per square foot range appear satisfactory. Optimum results from this preliminary step have been attained with the tinplate being made anodic under the above conditions for time periods of 0.1 to 5 seconds, preferably about 3 seconds.

Example 4 following describes the operation of the method of the invention in which tinplate strip was continuously coated using the foregoing anodic cleaning step in combination with the cathodic coating step.

EXAMPLE 4 A number of tests were conducted in conjunction with a tinplating line to evaluate the efficiency of the method of the invention on a continuous basis. Refiowed and unrefiowed tinplate strip of 0.125, 0.25, 0.50, 0.75 and 1.0 lb./bb. weight were processed at a line speed of about 500 feet per minute by a sequential operation in which the strips were first made anodic at a current density of 45 amperes per square foot in an aqueous solution at 24 C. of sodium carbonate at a concentration of 21 g.p.l. and then cathodic at a current density of 85 amperes per square foot in an aqueous solution having 17.5 g.p.l. of sodium dichromate and 11 g.p.l. chromium trioxide. Although the concentration of chromium trioxide relative to sodium dichromate was high; i.e. the sodium dichromate to chromium trioxide ratio was about 1.5:1, it was found that the anodic pretreatment was sufliciently effective to overcome the deviation from the preferred range in the cathodic treatment.

The following tests, Examples 4(a) to 4(i), indicate the efficiency of the coating produced on tin-base alloy surfaces of various tinplate weights. Specimens of coated tinplate from the line trial, together with specimens of tinplate having conventional passivation treatment only, were immersed for various periods of time in de-tinning mediums held at 122 C. and 15 p.s.i.g.

EXAMPLE 4(a) Tinplate coated according to the method of the invention was immersed for the 18 hours in vegetable soup for comparison with unaged and aged tinplate passivated by conventional coating techniques. The coating of the present invention proved markedly superior.

EXAMPLE 4(b) A test was conducted under the same conditions as the foregoing test with the time of immersion reduced to 3 6 hours. The coated tinplate according to the present invention in 0.25, 0.50, 0.75 and 1.00 lb./bb. weights did not show any evidence of detinning. Tinplate having 0.125 lb./bb. matte and reflowed surfaces proved as satisfactory as 0.25 lb./bb. conventionally passivated tinplate.

EXAMPLE 4 (c) Tinplate coated according to the method of the invention was subjected to a three-hour immersion in vegetable juice (V8 juice). This test indicated that the coating of the present invention was much superior to conventionally passivated tinplate of equivalent tin coating weight.

EXAMPLE 4 (d) Tests conducted under the conditions of the foregoing Example 4(0) for an eight-day immersion period indicated the tinplate having a coating of the invention was only slightly de-tinned whereas conventionally passivated tinplate was de-tinned severely. Tinplate of 0.125 lb./bb. refiowed material coated according to the method of the invention had tin remaining over of its surface at the termination of the eight-day immersion.

EXAMPLE 4(e) Tinplate coated according to the method of the invention immersed for three hours in pork and bean soup proved to have improved de-tinning resistance as compared to conventionally passivated tinplate of equivalent tin coating weight. Tinplate having a 1 lb./bb. coating conventionally passivated showed a noticeable etch whereas material coated according to the invention showed no evi dence of etching even on a 0.50 lb./bb. weight.

EXAMPLE 4(f) An immersion for twenty-five days in beer indicated no noticeable etching on tinplate coated according to the method of the invention whereas conventionally passivated tinplate was de-tinned extensively. Tinplate having only 0.125 lb./bb. coating showed no evidence of de-tinning at the termination of the twenty-five day immersion.

EXAMPLE 4 (g) A twenty-five day immersion in Coca-Cola indicated no observable difference between tinplate coated according to the method of the invention and conventionally passivated tinplate.

EXAMPLE 4(b) A two-week immersion in commercial carbonated drinks such as grape, pineapple-grapefruit, Florida punch and orange drinks indicated that tinplate. passivated according to the method of the invention was superior to conventionally passivated tinplate in each carbonated drink.

The foregoing tests confirmed that tinplate passivated according to the method of the present invention has excellent detinning resistance to media such as vegetable soup, vegetable juice and carbonated drinks as compared to conventionally passivated tinplate.

EXAMPLE 4(i) Abrasion tests were conducted on the tinplate coated according to the method of the invention and on conventionally passivated tinplate of equivalent tin coating weight. The tinplate according to the method of the invention proved superior to that conventionally passivated. It was noted from the foregoing tests that tinplate of 0.50 to 0.75 lb./bb. weight coated according to the method of the invention provided optimum abrasion resistance under normal production conditions. This abrasion resistance was superior or equivalent to conventionally passivated tinplate of 1 and 1.35 lbs./bb. weight. It is believed that the coating weight between 0.50 and 0.75 lb./bb. provides an effective bond between the complex surface conversion coating of chromium and tin oxides and the tin/iron interface, thus increasing the abrasion resistance.

7 EXAMPLE 4 1 Standard sulphide stain tests conducted on tinplate specimens of the present invention and conventionally passivated material indicated that the coating of the present invention improved sulphide stain resistance considerably.

The present invention provides a number of important advantages. Tinplate of various weights as low as 0.125 lb./bb. coated according to the invention shows surprisingly effective de-tinning and stain resistance while coated tinplate within the range of from about 0.50 to about 0.75 lb./bb. also shows unexpected-resistance to abrasion. Tinplate coated according to the method of the invention clearly shows improved resistance, throughout the range of tinplate weights as compared to conventionally passivated tinplate, to stain, corrosion and abrasion, while providing an improved base for lacquer, enamel or other organic-base coatings.

It will be understood, of course, that modifications can be made in the preferred embodiments of the invention described and illustrated herein without departing from the scope and purview of the appended claims.

What we claim as new and desire to protect by Letters Patent of the United States is:

1. A method of providing stain and corrosion resistant coatings on tin or tin-base alloy surfaces comprising the steps of making the surfaces cathodic for at least 0.2 second at a current density of at least about 20 amperes per square foot in an aqueous solution at a temperature of from about 18 C. to about 100 C. containing alkaline ions selected from the group consisting of ammonium ions and alkali metal ions; and dichromate and chromate ion in amount stoichiometrically equivalent to from about to about 30 grams per litre of sodium dichromate and from about 3 to about 6 grams per litre of chromium trioxide, and maintaining the ratio of sodium dichromate to chromium trioxide within the range of from about 4:1 to about 7:1 and the pH of the solution below 2.

2. A method of treating tin or tin-base alloy surfaces for rendering said surfaces stain and corrosion resistant comprising the steps of making said surfaces anodic for at least 0.1 second in an aqueous solution containing from about 0.5 to about grams per litre of an alkaline carbonate selected from the group consisting of ammonium carbonate and alkali metal carbonates at a current density of from about 20 to about 40 amperes per square foot, and making the surfaces cathodic for at least 0.2 second at a current density of at least 20 amperes per square foot in an aqueous solution at a temperature of from about 18 to about C. containing alkaline ions selected from the group consisting of ammonium ions and alkali metal ions; and dichromate and chromate ions in amount stoichiometrically equivalent to from about 15 to about 30 grams per litre of sodium dichromate and from about 3 to about 11 grams per litre of chromium trioxide, and maintaining the ratio of sodium dichromate to chromium trioxide within the range of from about 1.521 to about 7:1 and the pH below 2.

3. In a method as claimed in claim 2, said aqueous solution containing alkaline, dichromate and chromate ions in amount stoichiometrically equivalent to from about 15 to about 30 grams per litre of sodium dichromate and from about 3 to about 6 grams per litre of chromium trioxide, and maintaining the ratio of sodium dichromate to chromium trioxide within the range of from about 4:1 to about 7: 1.

4. In a method as claimed in claim 3, said alkaline ion being selected from the group consisting of sodium, potassium and ammonium.

5. In a method as claimed in claim 4, maintaining the pH of said aqueous solution about 1 by the addition of chromium trioxide.

6. In a method as claimed in claim 5, making said surfaces cathodic at a current density of from about 20 to about 50 amperes per square foot.

7. In a method as claimed in claim 2, said alkaline carbonate being sodium carbonate.

8. In a method as claimed in claim 2, said alkaline carbonate being sodium carbonate and said surfaces made anodic in the aqueous solution for about 3 seconds.

9. In a method as claimed in claim 2, said alkaline ion being selected from the group consisting of sodium, potassium and ammonium.

' 10. A hard, corrosion and abrasion resistant coating on tin or tin-base alloys of 0.50 to 0.75 lb./bb. weight produced in accordance with the method of claim 2.

References Cited UNITED STATES PATENTS 1,827,247 10/1931 Mason 204- 2,424,718 7/1947 Stevenson et al. 204-140 2,974,091 3/1961 Neish 204-140 3,313,714 4/1967 Joyce et al. 204140 ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 20435,- 140 

